Recently, power saving for electronic equipment is always required. In order to achieve the power saving, emphasis is laid on the reduction of electric power consumed by the electronic equipment, and the suppression of wasteful consumption of electric power by improving the efficiency of a power supply circuit itself which supplies a voltage to the electronic equipment. The power supply circuits widely use switching regulators capable of supplying input electric power with a high efficiency.
In switching regulators, a PWM control or a PFM control is widely known. In the PFM control, the control is performed such that the on time period of a switching element is made to be longer when the output voltage falls, and the on time period of a switching element is made to be shorter when the output voltage rises. In the PWM control, the control is performed such that the oscillation frequency of a drive pulse is made to be constant and the pulse width is changed in accordance with the load. Moreover, the PFM control further includes a method in which an on-timing is controlled at a certain fixed frequency, and the on-timing is skipped, thereby achieving a pseudo-PFM control operation.
As a control method of automatically switching between the PWM control and the PFM control, for example, Japanese Patent number 3647811, and Japanese Patent Application Publication number 2010-063276 disclose a configuration in which an operation is automatically switched from a PWM control operation to a PFM control operation or from a PFM control operation to a PWM control operation, on the basis of an error amplification output voltage generated from the difference between a partial voltage value of an output voltage from a switching regulator and a reference voltage from a reference voltage source.
Moreover, for example, Japanese Patent Application Publication Number 2008-092712 discloses a configuration in which a differential time generation means for forming a differential time signal indicating a differential time corresponding to a difference between the pulse width of a PWM control signal and the pulse width of a PFM control signal is included, and operation modes are switched by comparing the differential time signal with a reference signal for forming the PWM control signal in accordance with the differential time on the basis of the differential time signal.
Moreover, for example, Japanese Patent Application Publication number 2009-213228 discloses a configuration in which a circuit which counts the number of pulses of a PWM control signal and the number of pulses of a PFM control signal is included, and a reference of the number of pulses with respect to each of the number of pulses for shifting a mode from a PFM control to a PWM control and a mode from a PWM control to a PFM control is provided, in such a manner that switching the operation modes is controlled on the basis of a result of counting the pulses under the PFM control or a result of counting the pulses under the PWM control.
Moreover, for example, Japanese Patent Application Publication number 2009-225642 discloses a configuration in which a backflow current of an inductor current flowing from an output of a switching regulator to an output switch side is detected, and on the basis of a detection signal thereof, switching from a PFM control to a PWM control or from a PWM control to a PFM control is controlled.